The idea behind the therapy is that plasma of recently infected recovered coronavirus patients has antiviral antibodies that can be used to treat other patients with the virus.
“Convalescent plasma has a strong historical record of some efficacy during acute infectious pandemics,” study authors write.
The study, published on Wednesday in Mayo Clinic Proceedings, offered a larger analysis expanding on an initial report of 5,000 transfused patients. Patients were given the plasma through an expanded access program developed by the US Food and Drug Administration (FDA) and the Mayo Clinic.
Between April 3 and June 11 this year, 21,987 COVID-19 patients received the plasma transfusions. Nearly all of the patients had severe or life-threatening COVID-19 conditions.
Researchers said in the larger study, the overall frequency of serious adverse effects continued to be low, at less than 1 percent of all transfusions. The seven-day mortality rate dropped to 8.6 percent; the initial report of 5,000 patients showed 12 percent.
Some potential explanations for the drop in mortality were the U.S. health care community's improvement at managing hospitalized COVID-19 patients, and the blood banks’ greater availability of donated plasma, meaning that patients may now receive transfusions earlier in the course of treatment. The population of potential plasma donors has also grown, study authors said.
“In this regard it is remarkable that there was no system in place for convalescent plasma use in March 2020 and yet within months the nation is now able to meet most of the demand, despite complex logistics,” study authors wrote.
Finally, researchers postulated that since recovered COVID-19 patients were recruited faster for plasma donation, the plasma may contain higher levels of neutralizing antibodies.
Further research is needed to evaluate the potential explanations, researchers said.